Very wide question, but a few things to drain some extra battery I would for graphics use very big textures, multiple complex shaders, all the WorldEnvironment effects. For accessing nodes use searches based on names, maybe even from a regex based on a dictionary loaded from disk as often as possible, like directly in _process() without checking if it is needed or has changed. In if-statements test for the most detailed level first, instead of testing for the most general outcome first and then drilling down into more details, preferably looping over the complete list of nodes/lists. Update as much as possible of the scene and other nodes in every iteration of loops, instead combining each iteration and then after the loop updating other objects. That should do it
